Upgrading and dewatering of coal



United States Patent 3,381,388 UPGRADING AND DEWATERING 0F COAL Arun Kumar Chakravarti, Jagadish Chattopadhyay, Gour Gopal Sarkar, and Adiuath Lahiri, all The Central Fuel Research Institute, Jealgora, Bihar, India No Drawing. Filed Nov. 15, 1966, Ser. No. 594,358 9 Claims. (Cl. 34-9) This invention relates to the upgrading and dewatering of coal.

The production of some fine coal is inevitable in course of handling and preparation of coal, and its amount depends on the nature of the coal, the mining methods employed, the mode of coal handling, the degree of crushing and the scheme of preparation. When the raw coal is crushed to smaller sizes prior to washing for better recovery of the cleans, the production of fines increases considerably.

Various methods have already been proposed for upgrading and dewatering fine coal. One of these methods, known as the Convertol process was developed in Germany for simultaneous upgrading and dewatering of coal slurry. The process consists in dispersing the coal slurry (about 400-600 grams of solid per litre) by mixing with 15-10% of heavy oil in a specially designed mill. The oiled coal slurry is then passed through a centrifugal drier with a perforated basket of 0.2-0.5 mm. aperture, where the coal-oil mixture is retained in the basket and discharged as a concentrate, while the dirt particles pass through the centrifuge as eflluent.

Another method known as the Phase Separation Process, is based on the selective wetting and agglomeration of coal by petroleum fractions and has been applied for cleaning high ash content Indian coals. The agglomeration of Wetted particles is made use of for separating the oil-coal phase from the water-mineral phase by employing size separation techniques.

These and other conventional methods have various drawbacks such as:

(i) that they involve the consumption of large volumes of oil;

(ii) that the upgrading becomes effective only when the dirt particles are of fine sizes and of high ash contents;

(iii) that they require the use of special types of mills for intimately mixing the oil with the coal particles; and

(iv) that the slurry treatment by the conventional methods of slurry treatment by flotation technique followed by filtration cannot normally bring the moisture of the dewatered slurry down to below 25% unless the de watered slurry is thermally dried, which is a costly operation.

In short, the upgrading and subsequent dewatering of fine coal are the most expensive operations in a coal washing process. They require not only larger space, but also costlier equipment like vacuum filters and/ or thermal drier and expensive chemicals for upgarding by the standard and conventional methods of froth flotation circuit.

This invention has for its object improvements whereby the abovementioned drawbacks may be either eliminated altogether, or minimized considerably.

With this object in view, this invention broadly consists of an oleo-flotation process for upgrading and dewatering small coal, which process includes the steps of making the coal into a thick slurry, mixing the said slurry with a mixture of parafi'inic oil and tar oils, aerating the oily pulp so that the clean coal is rafted (floated up) as oiled flocs, removing the said oiled flocs. from the dirt, and dewatering them by centrifuging. This process will hereinafter be briefly referred to as the new oleo-flotation process.

In a preferred embodiment of this invention the new oleo-flotation process includes the following steps:

(i) Preparing a pulp of 35-40% solid content by thickening a coal slurry of size 1 mm.0 or 0.5 mm.0.

(ii) Mixing the pulp with a relatively high proportion of oils, 1.0-1.5 percent diesel oil or similar parafiinic oil and 0.05-0.2 percent of tar oils of a specific character (obtained from low or medium temperature carbonisation of coal), by weight of dry coal.

(iii) Diluting the oiled pulp with water to 20% solid consistency and rafting or floating up the oiled flocs by aeration in a cell.

(iv) Removing the oiled flocs and partially dewatering them on a suitable curved wedge-wire static screen.

(v) Mixing the product with the cleaned oversize (plus 0.5 mm.) in a paddle mixer or a pug mill or a screw mixer (in case of minus 0.5 mm.) and dewatering the resultant product in continuous basket centrifuge. For minus 1 mm. slurry, the partially dewatered product is directly centrifuged for final dewatering.

The distinguishing novel features of each of these five steps, in relation the step involved in the Convertol Process, will now be considered, step by step.

Firstly, step (i) is common to both the processes; but the Convertol process is restricted to coal particles of the top size 0.5 mm., whereas the top size may be as high as 1.0 mm. in the new oleo-flotation process.

Secondly, with regard to step (ii), the Convertol process uses an oil dose; but the oil dose is 2-10% and being mostly over 5 percent, whereas in the new oleo-flotation process, although the range of the dose has been mentioned as l5%, the usual effective dose is l2% using two types of oils instead of one as in the other process.

Depending on the slurry composition, i.e., depending on the characteristics of coal, necessity may, however, arise to use more than 2% oil, but in. no case the limit of 5% is exceeded. This low dosage has been possible by using a small quantity (ODS-0.2%) of L.T.C. tar oil fraction in the oil mixture.

Thirdly, in step (iii) of the new oleo-flotation process the oiled pulp is diluted to 20% solid and floated in a flotation cell. This operation diifers from the conventional flotation where a relatively much diluted pulp (540% solid consistency) is used. In the new process, dilution to 20% solid helps in retaining the major portion of oil with floated flocs which facilitates dewatering, handling, and also improving bulk density of the coke oven charge.

Fourthly, as regards step (iv) of the new oleo-flotation process, the floated flocs are partially dewatered on static wedge-wire screen. This operation is not present in any of the known processes e.g. in Convertol or Keroflotation.

Fifthly, as regards step (v), also used in the Convertol process, the oiled slurry is directly centrifuged for upgrading. In the new oleo-flotation process, only the floated concentrate from the cell is dewatered in the centrifuge after preliminary dewatering on a static screen and hence a lower capacity for centrifuge is required thus saving power and capital cost.

The partially dewatered flocs are mixed with the oversized cleans (plus 0.5 mm.) in a mixer and dewatered together in a continuous basket centrifuge, to give the final product. This step is absent in any of the hitherto known processes. The mixing of the coarser coal enables enhanced dewatering of even minus 0.5 mm. slurry.

Moreover, in a case where the top size of the fines (slurry) is 1 mm., the partially dewatered flocs are centrifuged independently in a conventional basket centrifuge and the dewatered product is then mixed with the dewatered cleans (1 mm.) to give the final product. Dewatering of oiled fines in the basket type centrifuge is a part of the process like Convertol, where the top size is restricted to 0.5 mm. or below. Although in the Convertol the centrifuge is specifically used both for the purposes of upgrading (by elimination of very fine particles of high ash content) and dewatering, the new oleo-flotation process employs the centrifuge only for dewatering the upgraded concentrates. Indian coal fines are generally low in ash unlike fines of coals in European countries. The Convertol process if applied to Indian slurry will only degrade the slurry instead of upgrading.

The new oleo-flotation process may be carried out as follows:

Crushed coal or natural smalls are wet screened at 1.0 or 0.5 mm. as the need may be. The undersized slurry from wet screening is thickened to 35 to 40% solid content in a static thickener with a rake or in a settling cone or by any other commercial method such as by thickening in hydro-cyclones. The thickened slurry is then treated with one to five (1-5) percent (by Wei ht) of a mixture of oils (a combination of diesel and tar oil). Normally 12% of oils suffice.

Due to surface forces involved and the nature of oils used, the cleaner fractions of the coal which are lyophobic form flocs and the dirt or dirtier fractions of coals remain largely as free slurry. The flocs thus formed, are mechanically floated or more truly rafted by aera tion in a conventional flotation cell. For flotation the solid consistency of pulp is kept at about 20% instead of 6 to 10% in normal froth flotation practice. The floated fiocs of both minus 1 mm. to or minus 0.5 mm. to 0 sizes are partially dewatered on a curved static wedge-wire screen and delivered by gravity to a pug mill or screw mixer or any other similar device for blending back with the oversize cleans (plus 1.0 or plus 0.5 mm. as the case may be). If the slurry be in the range of minus 1 mm. or coarser the blending back with coarser coal may not be necessary. The blended product or the partially dewatered slurry (minus 1 mm.) as the case may be is then taken to a continuous basket centrifuge of conventional design for dewatering. The flow-sheet of the process is outlined in the enclosed diagram.

Our process differs basically from the conventional process in that the slurry is mixed with comparatively higher doses of a combination of oils (of a special composition) then is normally required in conventional flotation process. As compared to Convertol or phase inversion process, the oil consumption is, however, much less (eg about 20% of that recommended for the Convertol process). The Convertol process for upgrading of the slurry is effective when the dirt particles are only of fine size and of high ash content. These conditions are not generally obtained in slurries produced from Indian coals having high inherent ash. Our process dispenses with costly filtration plants as well as with thermal drying operation which otherwise may be essential for reduction of moisture content of fine coal cleans to the desired limit when it is treated in the conventiona] way. Without thermal drying, slurries especially froth-floated slurr cannot be reduced below 20-25% moisture level.

The efiiciency of our new oleo-flotation process for upgrading of fines is generally comparable to that of standard froth flotation for slurries of Indian coking coal with the added advantage that it gives a product having a moisture considerably lower than that obtainable in the conventional process for fine coal treatment.

When the slurry is below 0.5 mm. in size, the moisture content of the centrifuged flocs is comparable to that obtained by vacuum filtration, but the former is easier to obtain. Efficiency of dewatering is higher when the fiocs obtained from minus 0.5 mm. slurry are centrifuged in a mixture with coarse coal. Slurries of minus 1 mm. size, on centrifuging alone, can yield products of moisture levels between 12-15% (compared to about 25% in normal practice). This product when blended with normally dewatered coarse coal, gives a charge of the desired acceptable moisture content.

The following examples illustrate the new oleo-flotation process in the form of typical Process Flow Diagrams. The said process, which is an integrated upgrading and dewatering process was carried out as follows:

Before upgrading, the raw coal is to be crushed to either 13 mm., 6 mm. or 3 mm. top size depending on the cleaning characteristics of the crushed coal. The crushed coal is to be deslimed to remove the fines below 1 mm. or 0.5 mm. prior to washing.

The oversized coal (plus 1 mm. or plus 0.5 mm.) will be upgraded in any conventional Washing system (e.g. cyclone or the like) to separate out cleans and sinks.

The coal slurry obtained during desliming operation as screen underflow containing below 1 or 0.5 mm. particles may account for 15 to 35 percent of the total coal depending upon the type of coal and the top size to which it is crushed. The slurry thus formed is to be thickened to 35 to 40 percent solid consistency. The thickened slurry is then conditioned with 1 to 5% (by Weight of dry coal) or a combination of diesel oil and low temperature tar oil fraction. For normal operation, an oil dose of 1-2% may be sufiicient.

The pulp is then diluted to 20% solid consistency and transferred to a flotation cell for conventional treatment. The floated fiocs (of size-fractions 1 mm. to 0 or 0.5 mm. to 0) is partially dewatered in curved wedge wire static screens.

The subsequent dewatering and treatment of cleaned flocs of different sizes (minus 1 mm. or minus 0.5 mm.) are effected separately in the manner as outlined below:

(a) Minus 1 mm. flocs The 1 mm. to 0 size partially dewatered flocs are taken directly to the basket type centrifuge for further dewatering. The oversized clean (plus 1 mm.) from conventional washer is also separately dewatered in basket type centrifuge. Then both the dewatered products (plus 1 mm. and minus 1 mm. cleans) are mixed in a mixing mill to get combined product (13 mm. to 0, 6 mm. to 0 or 3 mm. to 0) having below 8 percent surface moisture.

(b) Minus 0.5 mm. flocs The 0.5 mm. to 0 size partially dewatered fiocs are mixed in a mixing mill with the cleaned oversize (plus 0.5 mm. size) from conventional washer. The mixed coal (13 mm. to 0, 6 mm. to 0 or 3 mm. to 0) is then taken to a basket type centrifuge for final dewatering. The surface moisture of the dewatered product after this operation comes down to below 8 percent.

EXAMPLE I The general process as described above and the results obtained thereby are shown in the following process Flow Diagram No. 1:

3,381,388 5 6 Ergcess Flow Diagram No. l

Raw coal Crushed to 13 mm O or 6 mm O or 3 rfm 0 Wet screened iat l or 0.5 mm 1 I 1 13 1 1 mm O or 0.5 mm .5 mm o DZraded Thicliened to 35 40% solid. sinks Claims Conditioned with-oils.

x Oleo F1 ation I I 1 1 Rafted flees railings. I X r I 1 mm 0 I or 0.5 mm O f I \Y/ I I l I Centrifuge Centrifuge \Z Moirst. s.s-s.s% Moist. 12-14% 1 I I I I I Mixing Hill I Total dewatered roduct. Moist. 6 8% Mixing Mill X Centrifu e N.B. In case of minus 1 mm, 01' coarser size the slurry can be directly centrifuged or I? drained to a low moisture level (12 to 14%) but in the case of minus 0.5 mm. Slurry, T t d t product {t isl blended with coarser cleans for drainage or centrifuging to the desired moisture Moist. 6 8% EXAMPLE H 1 mm. and mixing of plus 1 mm. and mlnus 1 mm.

products after separate upgrading and dewatering. The The following Process Flow Diagram No. 2 relates moisture content of the final product (6 mm. to 0), in to the desliming of the crushed coal (below 6 mm.) at this case was recorded to be 7.5%.

Process Flow Dia rem Net scree1ed at 1 mm.

X 6 ma 1!. mm Minus 1 mm 2% 67.6 32.4 i 3.313;. 22.2

Cleaned Thickened to 40% solid I i Conditioned with oils.

Reje ens Cleans. Diesel Oil 1.0% I By wt. of

,. 33.8 '5 33.8 I..'r.c. Tar 011 0.05% 1 coal. (50%) (50%) I flees separaged in a flotation cell i F] I 11 .ocs. la in 's. 1 2%. 24.6 1% 7.8 1 (76.0%) (24.0%) )1 AS2156 1.3.3 Ashe; 51.1 I i centrifuged. Centrifu ged. Moist. 4.09% Moist. 12.

I T. I I Mixed. Dewatered total cleans (Minus 1.0 mm, 4273) Moist. 7.5%-

Figures within parenthesis are on individual size fraction basis,

7 EXAMPLE III The following Process Flow Diagram No. 3 relates to the desliming of the crushed coal (below 6 mm.) at 0.5 mm. and dewatering of combined cleans (6 mm. to in the same centrifuge. The moisture content of the final product (6 mm. to O) in this case was observed to be 7%.

Erocess Flow ldcgrarr 'No, 8

Raw coal (Jharia, XIII Seam) I Hot Screene? at: 0.5 mm

I i 6 0.5 mm Minus 0.5 mm 1% 83.6 16.4

I sent 21.8 I Thickened to solid. Cleaned Conditioned with oils. Diesel oil 1.0% .1 By wt. I L.T.C, Tar 0.05% I oi coal. Rejects Cleans 1% 41.8 2% 41.8 ilocs se arated by flotation cell 07 I I I I I Flocs. railings. I 1% 13.4 Y} 3.0 I (82.0%) (13.0%) I 451% 1 .5 Asa; 55.0 I I I I I Miiced. centrifuged. Dewatered total cleans (Minus 0.5 mm 25%) Moist. 7%

LLB. Figures within parenthesis are on individual size fr act ion basis.

What we claim is:

1. An oleo-flotation process for upgrading and dewatering small coal, which process includes the steps of making the coal into a thick slurry, mixing the said slurry with a mixture of paraffinic oil and tar Oils, aerating the oily pulp so that the clean coal is rafted (floated up) as oiled flocs, removing the said oiled flocs from the dirt, and dewatering them by centrifuging.

2. A process as claimed in claim -1, wherein the thick slurry is prepared as a pulp having about 35-40% solid content.

3. A process as claimed in claim 1, wherein the mixture of parafiinic oil and tar oils contains 1.0 to 1.5 percent of parafiinic oil and 0.050.2 percent tar oils, the said percentages being by weight of dry coal.

4. A process as claimed in claim 1, wherein the paraffinic oil used is diesel oil.

5. A process as claimed in claim 1, wherein the oiled pulp is diluted with water to 20% solid consistency before it is aerated for rafting (floating up) the coal as oiled flocs.

6. A process as claimed in claim 1, wherein the oiled flocs removed from the dirty pulp are partially dewatered on a curved wedge-wire static screen, before they are more completely dewatered by centrifuging.

9. A method of upgrading and dewatering small coal which comprises the following steps:

(a) preparing a pulp of 35 to 40 percent solid content by thickening a coal slurry of top size 1 mm.-0 to 0.5 mm.-();

(b) mixing the said pulp with a mixture of paraflin oil and tar oil obtained from low or medium temperature carbonisation of coal;

(0) diluting the oiled pulp with water up to 20% solid consistency, and rafting (floating up) the oiled fiocs by aeration in a cell;

(d) removing the oiled flocs and partially dewatering them on a curved wedge-wire static screen; and

(e) dewatering the resultant product in a continuous basket centrifuge.

References Cited UNITED STATES PATENTS 1,960,917 5/1934 Nagelvoort 34-9X FREDERICK L. MATTESON, JR., Primary Examiner.

JOHN J. CAMBY, Assistant Examiner. 

1. AN OLEO-FLOTATION PROCESS FOR UPGRADING AND DEWATERING SMALL COAL, WHICH PROCESS INCLUDES THE STEPS OF MAKING THE COAL INTO A THICK SLURRY, MIXING THE SAID SLURRY WITH A MIXTURE OF PARAFFNIC OIL AND TAR OILS, AERATING THE OILY PULP SO THAT THE CLEAN COAL IS RAFTED (FLOATED UP) AS OILED FLOCS, REMOVING THE SAID OILED FLOCS FROM THE DIRT, AND DEWATERING THEM BY CENTRIFUGING. 